Now experimenting with the magic of Fusion's Chao Editor, I have finally been able to see what actually increases Luck/Intelligence. Pretty sure this information has been plastered somewhere around here, and there have been discussions that both Chaos Drives and animals DO in fact raise luck and intelligence, but I'd like to give you a list of exactly what you can use if you want to boost these stats in particular.

The best items available to you (under normal game conditions) are the following:

After testing all other fruits imaginable, Tasty fruits are the way to go. The seed costs 200 rings, which is much better value for money compared to 300 rings for a Mushroom, which only raises Luck.
Generally speaking, if you're raising your Chao's stats the regular way through giving Chaos Drives and Animals, you shouldn't have much problem raising Luck/Intelligence too. If you're worried that your Chao may be lacking in some luck, opt for Chaos Drive instead of animals.

Unused items:
Mushroom x2, just gives double of what regular Mushroom gives you (+24 in Luck).
Smart fruit is by far the most potent, just one bite gives a staggering +100 (an instant level up) in Intelligence and +10 in Luck
Grapes gives +10 Luck

It seems that you may have looked at the effects of the fruits with Chao World Extended enabled. The effects of some of the fruits are altered by the main codes that it uses. It's what causes the Smart Fruit to increase the intelligence stat by one whole level per bite. I didn't know that the Tasty Fruit was altered as well, though.

There's actually two variations of the Mushroom. They both cost 300 rings and there's no telling which mushroom it is until your Chao eats it. One raises Stamina 50% and Luck 50% total. But the other raises Stamina 80% and Luck 100% total.

LimitCrown wrote:It seems that you may have looked at the effects of the fruits with Chao World Extended enabled. The effects of some of the fruits are altered by the main codes that it uses. It's what causes the Smart Fruit to increase the intelligence stat by one whole level per bite. I didn't know that the Tasty Fruit was altered as well, though.

I'm going to go spawn in some Tasty Fruit in SADXPC and see if FCE shows an increase in intelligence and luck. Might be able to discern if that's something CWE added or not.